Characterisation of the physico-chemical properties of surface-treated indium tin oxide anodes for organic light-emittin
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ABSTRACT We studied the surface properties of indium-tin oxide (fTO) modified by wet (aquaregia, ultrasonication, RCA) and dry (oxygen- and argon-plasma) treatments. The surface modification was investigated by surface energy, surface morphology, sheet resistance, carrier concentration, carrier mobility, and workfunction measurements. We report that the studied oxygen-plasma treatment induces: the highest surface energy with the highest polarity, the smoothest surface, the highest carrier density but the lowest mobility, the lowest sheet resistance, and the highest workfunction (stable in air). Polymer light-emitting diodes fabricated with the oxygen plasma treated substrates give the best performance in terms of electroluminescence efficiency and device lifetime. This is attributed to a favorable surface modification of fTO anodes by oxygen-plasma. INTRODUCTION Although a large variety of transparent conducting oxides (TCO) are known, such as tin oxide, indium oxide, zinc oxide and their combinations, the anode of Organic Light-Emitting Diodes (OLEDs), is in the vast majority of cases a thin layer of a mixed indium-tin oxide (ffO: 10 mol % tin oxide in indium oxide) or fluorinated tin-oxide
(FTO). ITO has been by far the common choice, due to the availability, its good transparency and low resistivity, and also to the ease with which it can be patterned. Due to a large variety of preparation and post-deposition processes, however, it is not clear to what extent a commercially available ITO glass may be suitable for LED fabrication. The electronic and chemical properties of ITO need to be understood and controlled in a more accurate way for use in LED applications. ITO has a large workfunction (4-5 eV, depending on the treatment), and it is then used for injection of holes into the Highest Occupied Molecular Orbitals (HOMO) of the conjugated semiconductors, rather than for injection of electrons into the Lowest Unoccupied Molecular Orbital (LUMO). For most polymers, however, the HOMO lies more than 5 eV below vacuum. A significant energy barrier to hole injection into the polymer is then expected to be responsible for an increase in the drive voltage of the devices. A possible strategy to ease the problem is to insert a hole injection-transport organic layer, and dramatic improvement of the lifetime and of the luminous efficiency have been reported by several groups [1-3] after coating a conducting polymer onto the
ITO anode, and in particular for poly(3,4-ethylene dioxythiophene), PEDOT, doped with poly(styrene sulfonate), PSS [4]. Surface treatments have also been proposed in order to improve efficiency, turn-on voltage, and lifetime of EL diodes [5-7]. In this paper we report a detailed study of the characterisation of the surface treatments on ITO anodes in terms of surface energy and morphology, carrier concentration and mobility, sheet resistance, workfunction, and chemical composition. 427 Mat. Res. Soc. Symp. Proc. Vol. 558 ©2000 Materials Research Society
EXPERIMENT Two different sets of processing tec
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